<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>task22</title>
    <style>
        .root{
            display:flex;
            padding:20px;
            width:1000px;
            height:300px;
            border:1px solid #000;
            margin:100px auto;
            margin-bottom:10px;
            justify-content:space-between;
        }
        .child_1{
            display:flex;
            padding:20px;
            width:450px;
            height:260px;
            border:1px solid red;
            justify-content:space-between;
        }
        .child_2{
            display:flex;
            padding:20px;
            widht:170px;
            height:220px;
            border:1px solid green;
            justify-content:space-between;
        }
        .child_3{
            display:flex;
            padding:20px;
            width:35px;
            height:180px;
            border:1px solid blue;
            justify-content:space-between;
        }
        input{
            marign-left:100px;
            width:50px;
            height:40px;
            font-size:15px;
            /*text-align: center;*/
        }
    </style>
</head>
<body>
    <div class="root">
        <div class="child_1">
            <div class="child_2">
                <div class="child_3"></div>
                <div class="child_3"></div>
            </div>
            <div class="child_2">
                <div class="child_3"></div>
                <div class="child_3"></div>
            </div>
        </div>
        <div class="child_1">
            <div class="child_2">
                <div class="child_3"></div>
                <div class="child_3"></div>
            </div>
            <div class="child_2">
                <div class="child_3"></div>
                <div class="child_3"></div>
            </div>
        </div>
    </div>
    <input type="button" value="前序">
    <input type="button" value="中序">
    <input type="button" value="后序">
</body>
<script src="task22.js"></script>
</html>